[expected.object.monadic] Add `typename` between `is_same_v<U​::​error_type, E>`

[EienMiku]

Mandates: U is a specialization of expected and is_same_v<U​::​error_type, E> is true.

Though U::error_type is obviously a type, typename is necessary.

is_same_v<U​::​error_type, E> should be is_same_v<typename U​::​error_type, E>.

[jwakely]

Yes, I think this is the Preserve typename Instantiating Template Arguments case, where it's still needed.